Source File Checks and Refills for Dtype Selective Zephyr Builds

[BujSet]

Summary

When Zephyr builds the portable operator library for a data type selective build, variables created during ExecuTorch's build are not transparent to the Zephyr build process. This PR introduces a check and refill it the source file lists for the portable operators are not visible to cmake. This fix will likely be helpful for other third party collaborators to use data type selective for integrating ExecuTorch with an external build system.

Test plan

Testing with manual changes to Zephyr CI tests. This PR checks out an executorch fork that already has this change. After applying the change, the zephyr model tests use data type selective build successfully.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/13211

• 📄 Preview Python docs built from this PR

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❌ 2 New Failures, 1 Unrelated Failure

As of commit 75f78ff with merge base 9b0feb0 ():

NEW FAILURES - The following jobs have failed:

• Build documentation / build (buck2) / Build doc (gh)
  At least one of the pre-conditions you specified did not hold
• pull / test-moshi-linux / linux-job (gh)
  RuntimeError: Command docker exec -t f3bc5036c94aeb22aba63d363c0a96c2f79840e5c5e308c30d3ecebd9eb9bf6e /exec failed with exit code 1

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / test-openvino-linux / linux-job (gh) (trunk failure)
  AttributeError: '_OpNamespace' 'quantized_decomposed' object has no attribute 'convert_element_type'

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[BujSet]

@pytorchbot label "release notes: none"

[BujSet]

Determined this should be something handled in the zephyr application code, not in the ExecuTorch repo.